1. Field of the Invention
The present invention relates to a water based ink composition for a rollerball pen. In particular, the present invention relates to an ink composition having shear thinning viscosity, which is used for a water based gel ink rollerball pen. The water based gel ink rollerball pen is structured so that one end is packed with a rollerball pen tip and the other end is packed with an ink backflow preventing body that follows ink in accordance with ink consumption, wherein ink is directly filled into an ink tank.
2. Description of the Related Art
Conventionally, among water based inks, as for an ink that has shear thinning viscosity, a technique relating to a water based ink composition for a rollerball pen, in which the ink composition includes a shear thinning viscosity-providing agent such as macromolecular polysaccharide, has been disclosed. (For example, refer to Japanese Patent Application Publication (Laid Open) No. Tokukaihei 4-214782, hereinafter referred to as patent document 1.)
This type of water based ink has shear thinning viscosity. When the ink is stably placed without being applied with a shearing force, this type of water based ink has high viscosity and is stably stored in an ink tank. However, when a rollerball pen including such ink is used for writing, a shearing force caused by a ball rotating with high speed makes the ink in the vicinity of the ball to have lower viscosity. As a result, the ink is discharged from a gap between the ball and a ball storage section, to be transferred onto a paper surface. The ink transferred to the paper surface is released from the shearing force and thus has high viscosity again, thereby preventing line-blurs of handwriting, which has been a disadvantage of a conventional water based ink composition.
Furthermore, the above-described shear thinning viscosity can prevent ink leakage, and no flow rate adjustment member for adjusting ink flow rate (e.g., pen core) is required. Therefore, there is an advantage that a writing instrument having a simple structure can be obtained.
The writing instrument using the above-described water based ink having shear thinning viscosity has a simple structure. In contrast to a conventional writing instrument using the flow rate adjustment member, the writing instrument using the above-described water based ink having shear thinning viscosity prevents air from entering the structure in accordance with ink consumption. Thus, an advantage is obtained in that a change in internal pressure is small and excessive ink discharge is prevented. However, this writing instrument requires a special care for preparation and filling of the ink. The writing instrument using water based ink having the above-described shear thinning viscosity is structured so that rear end face of the ink is packed with grease-like ink backflow preventing body that prevents solvent in the ink from evaporating, or prevents ink back flow when a writing tip end faces upward (upright status). This ink backflow preventing body follows the ink in accordance with ink consumption and is hereinafter referred to as liquid plug. This allows the ink to exist in an airtight space blocked from outside air. When gaseous matter is mixed into the ink, the gaseous matter coagulates with time and generates bubbles. Such bubbles cause an adverse influence on ink discharge during writing, generates bubbles at a writing tip end, and cause a risk of writing defect when the bubbles move.
In order to solve the above problems, a defoaming processing by a centrifugal separation has generally been performed when a rollerball pen is assembled. However, in a case where only this processing is conducted, the deforming is insufficient and thus gas may be left in the ink. When insoluble colorant such as pigment, metal powders, and titanic oxide is used, and the ink is subjected to a centrifugal defoaming processing with a strong G, a defect is arises in which the colorant exists unevenly in an ink storage container.
As for improvement of the above processing, a method has been suggested in which during assembly of the rollerball pen, the ink is subjected to a centrifugal defoaming processing under a reduced pressure (For example, refer to Japanese Patent Application Publication (Laid Open) No. 2004-58615, hereinafter referred to as patent document 2.).
However, this method requires a large apparatus and a long processing time, arising a problem in the production.
In order to improve the above defect of the production method, a method has been considered in which gaseous matter in ink is chemically removed. For example, an attempt to add antioxidant such as ascorbic acid or derivatives thereof, catechin derivatives, and choline or its derivatives) (For example, refer to Japanese Patent Publication No. Tokukouhei 7-113101, hereinafter referred to as patent document 3, Japanese Patent Application Publication (Laid Open) No. Tokukaihei 5-86319, hereinafter referred to as patent document 4, and Japanese Patent Application Publication (Laid Open) No. 2000-71676, hereinafter referred to as patent document 5.) and an attempt to add reducing agent such as sodium sulfite and pyrogallol (For example, refer to Japanese Patent Application Publication (Laid Open) No. 2002-80770, hereinafter referred to as patent document 6.) are disclosed.
However, among the compounds for chemically removing gaseous matter in ink, antioxidant (e.g., catechin derivatives, choline or its derivatives) has an influence on color material and causes discoloration, with respect to a system using dye. Furthermore, such antioxidant also has an adverse influence on dispersion with respect to a system using pigment. System using reducing agent also has a problem in stability with lapse of time.
Patent documents 3 and 5 disclose an attempt to add ascorbic acid or derivatives thereof, in order to chemically remove gaseous matter in ink.
The rollerball pen that uses water based ink includes lubricant for the purpose of preventing a ball and a tip seating surface from wearing and the purpose of providing a smooth writing feeling.
Phosphate-ester based surfactant (For example, refer to Japanese Patent Publication No. Tokukouhei 1-13508, hereinafter referred to as patent document 7) and unsaturated fatty acid salts (For example, refer to Japanese Patent Publication No. Tokukouhei 5-54876, hereinafter referred to as patent document 8) are preferable for the above purposes and have been generally used.
Unsaturated fatty acid salts have a structure in which carboxyl group binds with metal and aliphatic group extends into water. This provides a rollerball pen with appropriate lubrication effect and writing feeling.
However, unsaturated fatty acid salts also have a strong surfactant effect due to this structure. Thus, generation of bubbles is intense and bubbles tend to remain in refill even after the centrifugal defoaming process during the assembly of the refill.
In order to solve this defect, addition of antioxidant (bubble suppressant) such as ascorbic acid has also been suggested. However, since bubbles are easily generated due to the structure of the lubricant, sufficient effect cannot be obtained if the added amount of the antioxidant is small.
Thus, a sufficient defoaming effect requires a large amount of ascorbic acid or derivatives thereof to be added. This has caused an adverse effect such as aggregation of pigment, precipitation of dye, and discoloration.
In contrast, phosphate-ester based surfactant used in the present invention not only has a favorable lubrication effect but also provides reduced bubbling. Therefore, it is suitable for a direct filling type water based gel ink rollerball pen, in which residual bubble arises problems.
Japanese Patent Application Publication (Laid Open) No. 2005-132885, hereinafter referred to as patent document 9, discloses a technique in which ascorbic acid or derivatives thereof is added as a bubble suppressant to a rollerball pen that uses water based ink.
However, the technique disclosed in the patent document 9 is related to an ink having a low viscosity, for improving writing performance of a rollerball pen that uses water based ink. This is different from the ink for a water based gel ink rollerball pen of the present invention which is added with shear thinning viscosity-providing agent.
Furthermore, phosphate-ester based surfactant used in the patent document 9 is structured so that hydrophobic group is one of styrenated phenol-type, which has a hard writing feeling and insufficient lubrication effect, in contrast with the present invention.
Furthermore, some ascorbic acid and derivatives thereof have problems as described above. Thus, mere addition of ascorbic acid or its derivatives is insufficient. For example, although L-ascorbic acid and its salts have a favorable bubble removal effect, some of them have a problem in that dispersion failure of pigments or remarkable discoloration may occur with respect to a system using a specific dye.